I don't know if it's a commentary on sex --> bad or if Joss is creating a structure where the thing we want most is the thing we can't have because the cost to the rest of the world is too great, and we lose the thing we want by taking it. It's depressing but sex, in this context, isn't about bad - it's about loss, which I think is a valid way to talk about sex and relationships; it's just going to be a very short-lived and angsty relationship.

I like this a lot. It goes well with Angel's forever struggle as well (for him it's really not about sex, it's about happiness, and that's what makes his story so bleak, even though I love it and him). Though it might not work for every case of sex-badness in the Buffyverse, I think it goes a long way on many of them (as well as other non-sex badnesses that I won't mention so as not to spoil :)).